It is known that impurities contained in an aqueous medium including fine solid matter can be removed by filtering the aqueous medium using a porous hollow fiber membrane composed of polyvinyl alcohol, and the hollow fiber membrane clogged with impurities can be regenerated for reuse by washing with an acid and/or alkali as disclosed in Japanese Patent Publication No. 37037/83.
However, the above-described method requires a large quantity of an acid and/or alkali every time the filter is regenerated, and, therefore, involves handling of a large quantity of a highly concentrated acid or alkali waste water. In addition, the extent of restoration of filterability achieved by regeneration with an acid or alkali is not totally satisfactory. Hence, this method is not satisfactory for industrial application.
When a reaction is carried out in an aqueous medium in the presence of a microbial cell, an immobilized cell, etc., as a catalyst, the resulting aqueous solution from which the catalyst has been removed by filtration sometimes has a slight turbidity. Such turbidity should be removed before merchandising of the aqueous solution as such or in the form of a concentrate. This turbidity is attributed to fine solids suspended in the aqueous solution, and it is not easy to remove this solid matter through conventional filtration. Removal of the fine solids filtration can be achieved only with a filter membrane having a pore size as fine as 1 .mu.m or less, but a membrane having such a small pore size is soon clogged, and thus has a short working life.
For removing fine particles, filterability may be improved by pretreatment of the aqueous solution, such as heat treatment and acid treatment, or addition of a coagulant to the aqueous solution. However, these techniques cannot be applied to substances that are easily polymerized or required to have high quality, such as acrylamide as described before, and removal of turbidity forms a particular subject. Although addition of a coagulant flocculates fine particles in an aqueous solution to improve filterability, a part of the coagulant added remains in the aqueous solution to deteriorate product quality.
Hence, it is keenly demanded based on industrial considerations that a clogged filter membrane be repeatedly regenerated at high efficiency so as to be used for a long period of time.